Japan had what was easily the most diverse submarine fleet of any nation in
the Second World War. These included manned torpedoes, midget submarines,
medium-range submarines, purpose-built supply submarines (many for use by the
Army), long-range fleet submarines (many of which carried an aircraft),
submarines with high submerged speed, and submarines that could carry multiple
bombers.

Because of the vastness of the Pacific, Japan built many boats of extreme
range and size, many of which were capable of cruises exceeding 20,000 miles and
lasting more than 100 days. In fact, Japan built what were by far the
largest submarines in the world, indeed, the only submarines over 5,000 tons
submerged displacement, or submarines over 400 feet in length until the advent
of nuclear power. These same boats were credited with a range of 37,500
miles at 14 knots, a figure never matched by any other diesel-electric
submarine. These large boats could each carry three floatplane bombers,
the only submarines in history so capable. Japan built 41 submarines that
could carry one or more aircraft, while the vast submarine fleets of the United
States, Britain, and Germany included not one submarine so capable.

During the Second World War, there were 56 submarines larger than 3,000 tons
in the entire world, and 52 of these were Japanese. Japan built 65
submarines with ranges exceeding 20,000 miles at ten knots, while the Allies had
no submarine capable of this feat. By 1945, Japan had
built all 39 of the world's diesel-electric submarines with more than 10,000
horsepower, and all 57 of the world's diesel-electric submarines capable of 23+
knots surface speed.

The Japanese navy also built submarines with the fastest underwater
speeds of any nation's combat submarines. They employed 78 midget
submarines capable of 18.5 to 19 knots submerged, and built 110 others capable
of 16 knots. As the war was ending they completed four medium-sized
submarines capable of 19 knots submerged. This exceeds the 17.5-knot
performance of the famed German Type XXI coming into service at the same
time. As early as 1938, Japan completed the experimental Submarine Number
71, capable of more than 21 knots submerged.

Japanese submarines employed the best torpedoes available during the
Second World War. The Type 95 torpedo used pure oxygen to burn kerosene,
instead of the compressed air and alcohol used in other nation's
torpedoes. This gave them about three times the range of their Allied
counterparts, and also reduced their wake, making them harder to notice and
avoid. The Type 95 also had by far the largest warhead of any submarine
torpedo, initially 893 pounds (405 kg), increased to 1210 pounds (550 kg) late
in the war. All Japanese torpedoes made during the war used Japanese Type
97 explosive, a mixture of 60% TNT and 40% hexanitrodiphenylamine. Most
importantly, the Type 95 used a simple contact exploder, and was therefore far
more reliable than its American counterpart, the Mark 14, until the latter was
improved in late-1943. Japan also developed and used an electric torpedo,
the Type 92. This weapon had modest performance compared to the Type 95,
but emitted no exhaust and, therefore, left no wake to reveal its
presence. Similar electric torpedoes were used by several nations.

Given their size, range, speed, and torpedoes, Japanese submarines achieved
surprisingly little. This was because they were mainly employed against
warships, which were fast, maneuverable, and well-defended when compared to
merchant ships. Japanese naval doctrine was built around the concept of
fighting a single decisive battle, as they had done at Tsushima 40 years
earlier. They thought of their submarines as scouts, whose main role was
to locate, shadow, and attack Allied naval task forces. This approach gave
a significant return in 1942 when they sank two fleet carriers, one cruiser, and
a few destroyers and other warships, and also damaged two battleships, one fleet
carrier (twice), and a cruiser. However, as Allied intelligence,
technologies, methods, and numbers improved, the Japanese submarines were never
again able to achieve this frequency of success. For this reason, many
argue that the Japanese submarine force would have been better used against
merchant ships, patrolling Allied shipping lanes instead of lurking outside
naval bases. Bagnasco credits the Japanese submarine fleet with sinking
184 merchant ships of 907,000 GRT. This figure is far less than achieved
by the Germans (2,840 ships of 14.3 million GRT), the Americans (1,079
ships of 4.65 million tons), and the British (493 ships of 1.52 million
tons). It seems reasonable that an all-out blitz of the American west
coast, the Panama Canal, and the approaches to Hawaii, New Zealand, Australia
and India would have caused the Allies more difficulty than did the naval
deprivations that were actually achieved. Losing a significant number of
merchant ships, and also needing to spread meager defenses even more thinly
along two coasts, would surely have had some substantial consequences for the
United States in 1942.

The Japanese did, of course, make some attacks on merchant shipping in the
Pacific and Indian Oceans, but these were the minority of missions.
Frequently, they waited for fleets that were never seen, supported spectacularly
brave but inconsequential reconnaissance flights, or toted midget submarines
about, all of which achieved rather less than was possible with so valuable a
resource as the Japanese submarine fleet. Worse from a naval perspective,
Japanese submarines were increasingly employed in running supplies to the
starving garrisons of isolated islands. The Japanese expended hundreds of
sorties in this way, which might have otherwise been used offensively against
the Allied war effort. A submarine's cargo capacity was much less than
that of a relatively inexpensive freighter. However, Japan was
understandably reluctant to let island garrisons starve. Additionally,
many practically unarmed submarines (including 26 built for Army use) were built
specifically for the supply role, consuming production resources as well.

For their disappointing achievements, Japanese submarines paid heavily.
Japan started the war with 63 ocean-going submarines (i.e., not including
midgets), and completed 111 during the war, for a total of 174. However,
three-quarters of these (128 boats) were lost during the conflict, a proportion
of loss similar that experienced by Germany's U-Boats. Most of the
surviving boats were either dedicated to training roles or were recently
completed and never saw combat. Of those which saw significant combat, the
toll was very grim indeed. For example, of the 30 submarines that
supported the Pearl Harbor attack, none survived the war.

Compared to German submarines, Japan's huge boats were relatively easy to
sight visually and with radar, slow to dive, hard to maneuver underwater, easy
to track on sonar, and easy to hit. Japanese hulls were also not as
strong as those of German boats, and therefore could not dive as deeply nor
survive such rough treatment. Also, they lacked radar until the first sets
were installed in June 1944, and never had sets as good as the Allies
possessed.

Compounding these deficiencies, Japan was at war with the United States and
the United Kingdom, two nations embroiled in a vast conflict with hundreds of
U-Boats in the Atlantic, and hence two nations which poured lavish resources
into anti-submarine warfare (ASW) research and development. As an example
of the fruits of this research, in June 1944 the US Navy sank the I-52 by using
code-breaking to discover her schedule, finding her at night with radar-equipped
carrier-based aircraft, tracking her underwater with sonobuoys dropped by those
aircraft, and sinking her with acoustic homing torpedoes dropped by the same
aircraft. The Japanese could achieve none of these technological feats at
that time.

In the face of such disadvantages, morale declined within the Japanese
submarine force. This is reflected in a post-war report prepared by the US
and British Navies which states, "It was frankly impossible to believe that
submarines could spend weeks on the US west coast 'without contacts,' or spend
more than 40 days running among the Solomons during the Guadalcanal campaign
'without seeing any targets.' Even the Japanese commanding officers could
not disguise their embarrassment when recounting these tales. Further
enlightenment is found in the extremely large number of times the target was
'too far away to attack.'"

I am deeply indebted to Mr.
Forest Garner for taking the time and effort to tackle this section of the
site, which has been badly neglected by me since time immemorial. Thanks also go to Bob Hackett
for his additions to this page.